I think all these things are possible. Not necessarily this year to win this year, though if they remain in the mix when the time comes, they'll be active.
A SP who is not a pending free agent is very possible (though Marcus Stroman appears to be off the table these days).
A veteran bullpen arm seems almost inevitable though the internal situation seems pretty good there (a lot depends on Neshek, and possibly on how things play out with Neris).
Hamels would provide depth (it's still optimistic to think everyone in this group will stay both good and healthy all season) and come playoff time when you only use three-four starters and the third-time-through-the-order thing becomes ironclad, several of your starters will be relievers anyway.
I do think they could acquire another catcher even if it's just for defense. Knapp could be optioned and then you make a permanent decision in the off-season.
The OF does seem set, obviously. I don't know it's set for 2019 or 2020 but it doesn't seem like an area you'd upgrade just for this season. I would still include Williams or Quinn or Cozens in a trade, potentially (but not for a rental).
I think Machado is a special case. If you go after him you have to include the value of "recruiting" him. You are basically getting an exclusive negotiating period too.
If you've got a shot to win the division you gotta take it. A wild card race is a different story.
Of course you don't want to trade your top prospects or too much depth but top prospects rarely actually move at the deadline (Top 5, yes, Top 3 no). And as down as we are on the three first-rounders and a few other pitchers currently they'd still have some value. Look at what we got for Lee. Look at what the Yankees and Astros gave up for Gray and Verlander. None of those deals were outrageous for the buyer.